A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be applied to a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
A light-emitting element is said to have the following light emission mechanism: when voltage is applied between a pair of electrodes with an EL layer including a light-emitting substance provided therebetween, electrons injected from the cathode and holes injected from the anode are excited in a light emission center of the EL layer, and energy is released and light is emitted when the excited state returns to a ground state. The excited states generated in the case of using an organic compound as a light-emitting substance are a singlet excited state and a triplet excited state. Luminescence from the singlet excited state (S1) is referred to as fluorescence, and luminescence from the triplet excited state (T1) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S1: T1=1:3.
Therefore, the EL layer of the light-emitting element includes a host material and a guest material (a phosphorescent compound), whereby the light-emitting element can have an element structure that utilizes phosphorescence as well as fluorescence and element characteristics can be improved (e.g., see Patent Document 1).
Further, the EL layer includes a hole-injection layer, a hole-transport layer, a light-emitting layer, an electron-transport layer, an electron-injection layer, or the like, and includes at least a light-emitting layer. Note that materials suitable for respective functions of these layers have been developed to be applied, whereby element characteristics are improved (e.g., see Patent Document 2).